1. Field of the Invention
The present invention relates to a printed circuit board (PCB), and in particular to a printed circuit board routing with good signal quality and less electromagnetic interference (EMI) effect.
2. Description of the Related Art
Along with rapid advances in printed circuit board (PCB) technology, electromagnetic interference (EMI) problems have increased in severity. When semiconductor devices have higher speeds and higher device densities, noise occurs. Therefore, for printed circuit board designers, electromagnetic interference (EMI) problems have become more and more of a challenge.
FIG. 1a shows a local top view of the conventional two-layered printed circuit board 300 showing routing of power and signal planes. FIGS. 1b to 1c show cross sections along lines A-A′ and B-B′ of FIG. 1a, respectively. The conventional printed circuit board 300 has a top layer on a top surface 102 of a substrate 100, covered by a solder mask layer 126. The top layer comprises power traces 108a and 108b and a signal trace 112. The conventional printed circuit board 300 also has a bottom layer covered by a solder mask layer 130 comprising a ground plane 140 on a bottom surface 103 of the substrate 100. For descriptive convenience, the solder mask layer 126 is not shown in FIG. 1a. The power traces 108a and 108b are used for power transmission, and the signal trace 112 is used for signal transmission. As shown in FIG. 1a, the signal trace 112 substantially along a second direction 304 may form a power transmission barrier between the adjacent power traces 108a and 108b substantially along a first direction 302, which is not parallel to the second direction 304. As shown in FIG. 1b, for power transmission between the adjacent power traces 108a and 108b, a conductive layer 108c is formed on a bottom surface 103 of the substrate 100. The conductive layer 108c is respectively electrically connected to the power traces 108a and 108b by via plugs 134 through the substrate 100, and isolated from the ground plane 140 by a split 150. As shown in FIGS. 1a and 1c, the signal trace 112 passes over the split 150 surrounding the conductive layer 108c. When signals, especially high-speed signals, are transmitted along the signal trace 112, however, the current return path of the high-speed signals not only remains under the signal trace 112, but also along the split 150 directly under the signal trace 212. Therefore, the longer current return path results in higher impedance and signal attenuation problems. Also, the current return path along the split 150 may generate an undesired magnetic field vertical to the directions 302 and 304. The undesired magnetic field increases the coupling coefficient between adjacent signal traces and exacerbates electromagnetic interference (EMI) problems. A multi-layered PCB, which separates power, signal and ground planes in different layers, may be used to mitigate the aforementioned problems, but adding layers will increase the manufacturing cost of the PCB.
Therefore, a printed circuit board routing with good signal quality and less electromagnetic interference (EMI) effect is desirable.